1. Field of the Invention
The present invention relates to transversal surface acoustic wave filters in which an input transducer and an output transducer are arranged on the surface of a surface acoustic wave substrate, and more particularly, to a surface acoustic wave filter in which each of the input and output transducers includes a plurality of oblique electrode fingers having an electrode finger pitch that changes along a direction that is substantially perpendicular to a surface acoustic wave propagation direction.
2. Description of the Related Art
Outstanding in-band flatness and selectivity (steepness of a shifting area shifting from a pass band to an out band) are required for intermediate-frequency (IF) filters for satellite digital audio radio service (SDARS) tuners. Also, a wide variety of filters, such as those ranging from relatively narrow-band filters having a band width ratio of approximately 2% to very wide-band filters having a band width ratio of approximately 20%, are required.
PCT Japanese National Phase Patent Publication No. 11-500593 discloses a method for increasing the bandwidth of a surface acoustic wave filter. In this method, so-called slanted transducers are used. In the slanted transducers, an electrode finger pitch of input and output transducers changes along a direction perpendicular to a propagation direction of surface acoustic waves. In other words, electrode fingers extend in an oblique direction with respect to the direction perpendicular to the propagation direction of surface acoustic waves. A transversal surface acoustic wave filter using slanted single-phase unidirectional transducers (SPUDTs) is described in PCT Japanese National Phase Patent Publication No. 11-500593. Since the input and output transducers are slanted transducers, an electrode finger pitch is different in the direction perpendicular to the surface acoustic wave propagation direction. Thus, outstanding in-band flatness and a wider bandwidth is achieved.
However, in order to increase the selectivity of the surface acoustic wave filter described in PCT Japanese National Phase Patent Publication No. 11-500593, the number of pairs of electrode fingers must be increased. Thus, the size of the surface acoustic wave filter is increased. In addition, when the number of pairs of electrode fingers increases, the angle of inclination of electrode fingers near ends thereof in the surface acoustic wave propagation direction is increased. Thus, a direction of radiation of surface acoustic waves is greatly shifted from the surface acoustic wave propagation direction, and this deteriorates the characteristics of the surface acoustic wave filter.